For the owners of 600s and 65s with rear A/C, I have often wondered... rarely is there a passenger in the rear seats of my car so the A/C remains off. After some thought, I wondered if this will cause the seals in the rear unit to fail prematurely and if I would be better off letting it sync with the front?

Thoughts?

I would say sync to the front, or keep the rear set to a moderate fixed temperature. That way the system isn't working harder to maintain three or four different temps, but you have the whole system working to heat or cool the cabin rather than making the front system work harder. I have never to my recollection seen a a mechanical or electromechanical system that benefits from disuse.